1. Field of the Invention
This invention relates to small propulsion systems for maneuvering spacecraft and, more particularly to an electrothermal arcjet thruster having an anode with a centrally positioned constrictor. This constrictor converges from an upstream end to a downstream end. As the result, both the tangential and axial velocity of a propellant gas increase as the gas flows through the converging constrictor.
2. Description of the Prior Art
An electrothermal arcjet thruster converts electrical energy to thermal energy by heat transfer from an arc discharge to a flowing propellant. The thermal energy is converted to directed kinetic energy by expansion of the heated propellant through a nozzle.
Most electrothermal arcjet thrusters have as common features an anode in the form of a nozzle body and a cathode in the form of a cylindrical rod with a conical tip. The nozzle body has an arc chamber defined by a constrictor in a rearward portion of the body and a nozzle in a forward portion thereof. The cathode rod is aligned on the longitudinal axis of the nozzle body with its conical tip extending into the upstream end of the arc chamber in spaced relation to the constrictor so as to define a gap therebetween.
When a sufficiently high current is applied between the anode and the cathode, an electric arc is initiated. When the arc is initially struck, the electric current travels the path of smallest inductance (path of least resistance) between the cathode body (this portion of the arc is referred to as the "arc root") and the anode (this portion of the arc is referred to as the "arc foot").
In a typical arcjet configuration, the arc is believed to originate on the cathode shoulder, upstream of the cathode tip. Gas dynamic forces in concert with thermal effects, cause the arc root to move toward the cathode tip. At the same time, the arc foot is swept along the anode towards the anode portion of minimum cross-sectional area, the constrictor. Stable arcjet operation is achieved when the arc foot passes through the constrictor and is distributed diffusely, with a low energy density, along the diverging walls of the anode.
When in a stable operation mode, the propellant gas is heated in the regions of the constrictor and of arc diffusion at the mouth of the nozzle downstream from the constrictor. The super-heated propellant gas is exhausted out from the nozzle to achieve thrust.
Historically, propellants, such as ammonia or hydrogen, have been used in electrothermal arcjet thrusters. More recently, hydrazine (N.sub.2 H.sub.4) has been used. Propellants such as ammonia and hydrazine are preferred because these propellants are storable as a liquid without refrigeration while cryogenic propellants such as hydrogen and helium are not. The liquid storable fuels are converted to a gaseous propellent by passing the fuel through a gas generator.
The specific impulse (I.sub.sp) determines the propellant mass required to complete a flight. I.sub.sp is denoted in pounds of force-second per pound of mass. The generation of a high I.sub.sp in an arcjet thruster requires operation of the thruster at a high specific energy (as denoted in watts/kg). The cryogenic propellants have a typical I.sub.sp value of up to 1,500 lbf-sec/lbm. The liquid storable propellants have a much lower specific impulse, on the order of 800-1000 lbf-sec/lbm.
One way to increase I.sub.sp is to increase the thrust efficiency of the arcjet thruster. U.S. Pat. No. 5,111,656 to Simon et al. discloses increasing the specific impulse of a propellant by a unique nozzle configuration. The exhaust portion of the nozzle has a divergent recombination portion in tandem with a divergent expansion portion. The divergence of the recombination portion is less than that of the expansion portion, causing a temporary delay in the pressure reduction of the propellant gas. This delay creates a relatively high pressure region in the recombination portion of the nozzle permitting a partial recombination of the ionized and neutral species of the propellant gas and a partial recovery of frozen flow losses back into the gas.
U.S. Pat. No. 5,111,656 is incorporated by reference in its entirety herein. The biangle nozzle disclosed in that patent increases the efficiency of the electrothermal arcjet thruster at low power levels by reducing frozen flow losses. However, the nozzle also generates more heat at the anode surface and, as the energy level (power/mass flow rate) of the thruster increases, the advantage over a single angle nozzle decreases. At relatively high specific energy levels, the efficiency of a biangle nozzle is inferior to that of a single angle nozzle.
One method to limit the transfer of heat from the electric arc to the anode is disclosed in U.S. Pat. No. 4,800,716 to Smith et al. The Smith et al patent discloses forming a portion of the constrictor from an electrically insulating material such as boron nitride, alumina or berylia. However, the typical anode body is formed from tungsten or a tungsten alloy. Incorporating ceramic inserts into the metallic anode body increases both the complexity and the cost of manufacturing the anode. In addition, due to the high operating temperatures of the arcjet thruster, typically on the order of 1500K, coefficients of thermal expansion must be closely matched.
A different way to increase I.sub.sp is to reduce the mass flow rate while maintaining the power level. However, as the mass flow rate is reduced, the stabilizing gas dynamic forces diminish. Eventually, stable operation of the arcjet thruster ceases and the arc foot attachment moves upstream back through the constrictor to the point of smallest inductance along the converging upstream portion. Upstream arc attachment produces an unstable current/voltage condition, typically characterized by low voltage and high current requirements. Under these conditions, destructive energy densities are concentrated at the arc foot leading to erosion of the surface of the anode.
There exists, therefore, a need for an anode geometry permitting a reduction in the mass flow rate while inhibiting the upstream movement of the arc foot. Such an anode configuration would provide an arcjet thruster having a higher specific impulse with increased cathode and anode lifetimes.